Vibration absorbers have been effectively employed to reduce vibrations of a system or body. If a body is forced to vibrate at a frequency .omega..sub.1, an absorber k.sub.2, W.sub.2, tuned to the same frequency of .omega..sub.1 =k.sub.2 g/W.sub.2, introduce an opposing force equal to that of the disturbing force to suppress completely the vibratory motion of the body. Such absorbers are in wide use and generally are well known as FRAHM absorbers.
In such systems, a mass is suspended or connected to a vibrating body by a spring and the spring-mass system will have a resonant frequency of its own and if tuned to the same frequency as the vibration in the body to which it is secured may cause the vibratory motion of the body to be suppressed.
Such systems are of particular interest in the construction and operation of helicopters wherein many vibratory modes must be contended with. For example, of interest is the suppression of lateral vibration of the aircraft fuselage in its first lateral natural mode in response to rotor excitation. Helicopters are designed to operate the rotor system at a substantially constant normal operating speed of, for example, 348 RPM. This frequency of 5.8 Hertz is preferably displaced on a frequency scale from a natural frequency of the fuselage. FRAHM dampers thus installed on the aircraft are usually designed to absorb vibrations at multiple harmonics of the frequency of operation of the rotor system. However, the rotor system may not always operate at precisely the same frequency and it is therefore desirable to provide for absorbtion of unwanted vibration over a substantial frequency range.